The invention described below is about an apparatus for the measurement of dirt density on steel plates, that can be used in the plate manufacturing industry, in the manufacture of appliances and in any other manufacturing process involving steel plates, in which the degree of cleanliness of the plate is relevant.
The existing alternative methods to determine the dirt density on the surface of rolled steel plate are based on laboratory procedures carried out on plate samples. The most common one consists in sampling the dirt by means of a transparent adhesive tape. When the tape is placed in contact with the plate, the dirt (or part of it) sticks to the tape, modifying its optical transparency, which is subsequently measured at the lab. Alternative techniques are based on the quantitative analysis of the dirt removed by solvent extraction or by incineration and subsequent determination of the carbon residues.
None of the existing techniques allows the measurement of dirt in real time (i.e., at the time the dirt is sampled) and, much less, with the plate in motion.
The apparatus described below has the advantage of allowing the measurement during the final stages of plate inspection, and of providing a dirt density profile along the plate, and not only at points where the plate is sampled. Being an entirely automatic measurement, it is less subject to measurement errors originating from a manual operator.
The apparatus described is capable of measuring the density of the dirt that is present on the surface of rolled steel plate. Such measurements may be carried out directly on a moving plate, or on stationary samples. The apparatus consists of a pulsed laser, the optical devices to aim the laser beam at the plate to be measured, an acoustic detector (microphone) and the associated electronics for signal processing and control.
The operating principle is based on the fact that the surface dirt is constituted by a film of grease or oil and particles of different compounds. If a laser pulse of sufficient energy per beam cross sectional area and short duration (xcx9c10 nanoseconds) impinges on the plate, a sudden reheating and evaporation of the dirt is produced. During this process called xe2x80x9cablationxe2x80x9d particles are ejected along with oil and other residual fluids. Various phenomena are produced as a consequence of the interaction between the laser pulse and the film on the plate surface. One of them is the emission of a crisp sound, the intensity of which is a function of the amount of dirt present. This intensity is adopted as a measurement of the ejected dirt, which increases with the dirt density present on the plate. For this purpose, a microphone connected to a signal processing system is placed at an adequate distance from the point where the laser hits the plate. The signal processing scheme required is the determination of the amplitude of the acoustic signal.
The apparatus described is sketched in FIG. 1, whereas FIG. 2 shows the percentage of dirt measured as a function of the firing order (see below).